The present invention relates to the fabrication of electronic devices and, more particularly, to in-line soldering of electronic components onto circuit boards.
FIG. 1 shows, schematically, a portion of an assembly line for electronic devices. In a paste printer 10, solder paste is printed on circuit boards where leads of electronic components, such as packaged integrated circuit chips, are to be soldered to conductors on the circuit boards. In a pick and place station 12, components are placed in the appropriate locations on the circuit boards. In a reflow oven 14, the circuit boards, with the components in place, are heated by circulating hot air to melt the solder paste, thereby soldering the components onto the circuit boards. The arrows in FIG. 1 indicate the direction of movement of the circuit boards. This process typically is fully automatic.
FIGS. 2A through 2F illustrate six things that can go wrong in this process before the components are soldered to the circuit boards in reflow oven 14. FIG. 2A is a side view of a defective component 18 mounted on a circuit board 16 and ready for soldering. Two leads 20a and 20b are shown emerging from the side of component 18. Lead 20a is in contact with a corresponding solder paste bead 22a. Lead 20b is bent upwards and not in contact with a corresponding solder paste bead 22b. When circuit board 16 is heated in reflow oven 14, component 18 is incompletely soldered to circuit board 16. FIG. 2B is a plan view of another component 18' mounted on circuit board 16 and ready for soldering. Most solder paste beads 22 are in contact with only one lead 20' of component 18'. Solder paste bead 22' has been printed incorrectly by paste printer 10 and is in contact with two leads 20'. Solder paste bead 22' short circuits the two leads 20' with which it is in contact. FIG. 2C is a plan view of circuit board 16 where pick and place station 12 has failed to place a component in contact with solder paste beads 22. FIG. 2D is a plan view of component 18' translated with respect to solder paste beads 22. FIG. 2E is a plan view of component 18' rotated with respect to solder paste beads 22. FIG. 2F is a side view of two components 18' and 18" mounted one above the other on circuit board 16. Leads 20' of component 18' are in contact with solder paste beads 22, but leads 20" of component 18" are suspended above leads 20' and mask leads 20' and solder paste beads 20.
Because of assembly defects such as those illustrated in FIGS. 2A through 2F, circuit boards 16 are inspected upon emerging from reflow oven 14, either manually, or automatically in an automatic inspection station, and defective circuit boards 16 are either repaired in a rework station or discarded. This extra step of inspection outside of reflow oven 14 detracts from the productivity of the assembly line. There is thus a widely recognized need for, and it would be highly advantageous to have, an in-line apparatus and method for soldering electronic components to circuit boards that automatically inspects the solder joints as they are formed.